1. Field of the Invention
The present invention relates to a DS-CDMA (Direct Sequence-Code Division Multiple Access) base station asynchronous cellular system, and more particularly to an initial cell search method for a mobile station and a transmission power control method for a perch channel at a base station, which is combined with the initial cell search method.
2. Description of the Related Art
In recent years, the downsizing and the popularization of a cellular phone, etc. have been rapidly advancing with the size-reduction of a processor, etc. In a system accommodating such a cellular phone, a continuously moving mobile station must be accommodated in a suitable base station. At the same time, a system which accommodates mobile stations as many as possible is desired for the upcoming popularization of a cellular phone. However, since an available frequency bandwidth is limited with a conventional frequency division multiplexing technique, the number of mobile stations which can be accommodated is limited as a matter of course. Accordingly, close attentions are currently paid to a CDMA communication using a direct sequence. In the CDMA communication, a transmission signal is spread-modulated with a spread code which differs depending on each channel accommodated by a base station. On a receiving side, the transmission signal is regenerated by despreading the spread-modulated signal with the same code as that used by the base station. In this case, the reception signal must be multiplied by the despread code (the same as that used on the transmitting side) at suitable timing on the mobile station side, that is, the receiving side. Accordingly, to which channel of which base station a mobile station is to be connected is determined in the initial stage of the communication. At the same time, the multiplication timing of a despread code, which is intended to continuously connect the mobile station to that channel must be obtained. Namely, an initial cell search must be made.
The initial cell search is an operation for initially determining a visited cell of a mobile station (the visited cell is an area where a particular base station can accommodate a mobile station when the mobile station stays within the visited cell) when the mobile station power is turned on. At this time, the mobile station receives a perch channel transmitted from the base station, and attempts to obtain the information broadcast by the channel. The perch channel is a channel which helps a mobile station identify the despread code of the signal transmitted from a base station, or capture the channel transmitted to obtain despreading timing in the initial cell search.
In the system which is assumed by the present invention and will be described later, a perch channel is spread with a short code for synchronously capturing the perch channel, and a long code for identifying the channel from the base station. The perch channel is assumed to be further spread with a group short code indicating to which group the long code used for the perch channel belongs among many long codes so as to facilitate a long code search. Here, all of the short code, the group short code, and the long code are spread codes which respectively have their use purposes.
Since which long code is used for a certain downstream channel (a channel used for a communication from a base station to a mobile station) cannot be identified, it must be identified by examining the long code of a particular (perch) channel. Additionally, also the phase of the long code (the despreading timing when the long code is used in a communication) must be identified.
As the conventional initial cell search method of a DS-CDMA system with a control channel, which uses a long code which differs depending on each cell and a synchronization short code common to all cells, the technique disclosed by the Japanese Laid-open Patent Publication No. 10-126380 can be cited. With this conventional technique, the initial cell search for a single-frequency carrier wave signal can be made at high speed. Furthermore, as the technique obtained by further developing the above described conventional technique, “A High-speed Cell Search Method Using a Long Code Mask in DS-CDMA Base Station Asynchronous Cellular” recited in the Electronic Information Communication Society Research and Technical Report RCS96-122) exists. The format of the perch channel signal to which the above described techniques are applied is shown in FIG. 1.
FIG. 1 shows that a perch channel 100 signal is transmitted from the left to the right of this figure. A long code is intended to identify a channel accommodated by a base station. When a communication is made using the channel identified by the long code used by a certain base station, signals are transmitted and received by spreading and despreading the signals with this long code in all cases during a call. The perch channel signal is spread with the long code unique to the channel, and is further spread with a short code for synchronously capturing the perch channel 100 signal, which is common to all of base stations. The beginning portion of the long code, which is spread with the common short code, does not include a long code. The portion where no long code exists is further spread with a group short code indicating to which group the used long code belongs among many long code groups in addition to the common short code.
This initial cell search method is mainly composed of three stages. These stages are summarized below.    [First Stage] A destination base station whose reception power is maximized is determined by performing a correlational square amplitude operation between a reception signal and a short code, and by taking an average value of the correlational square amplitude operation. At the same time, slot synchronization is made. Here, the slot synchronization is the timing at which a despreading process is performed with the short code, the group short code, and the long code. Additionally, the correlational square amplitude calculation is an operation for calculating the correlation values for an I signal and a Q signal of a reception signal, and for squaring and adding the correlation values for the I signal and the Q signal, which are obtained by the above described calculation. This operation is equivalent to an operation for squaring the length of a vector when the correlation value of a signal is recognized to be the vector on an I-Q plane where the correlation values of the I and the Q signal are respectively indicated by the horizontal and the vertical axes. The reason that the average value of the correlational square amplitude calculation is taken is to suppress an influence of noise included in a correlation value.    [Second Stage] A group short code corresponding to a plurality of long codes is identified by using the slot synchronization timing established in the first stage. Used to identify the group short code is a method for calculating the correlation value of a reception signal with the group short code, and for determining whether or not the correlation value equal to or larger than a predetermined value is obtained. Long code candidates are limited at this stage.    [Third Stage] The long code synchronization and the long code of the perch channel are determined based on the result of the correlational square amplitude operation between the reception signal and the long code. The long code determination method is a method for calculating a correlation value with a reception signal by using both of the long code and the common short code, and for determining that the long code used for the perch channel is obtained when a predetermined or larger correlation value is obtained. If this process is unsuccessfully performed, the process goes back to the first stage and another long code candidate is used.
For the details of the conventional initial cell search method, please refer to the above described patent publication or technical document.
However, it is impossible to apply this technique to a DS-CDMA cellular system using a perch channel of a multiple-carrier-frequencies signal as it is. This is because perch channels exist at a plurality of frequencies, and the operation for receiving all of the frequencies is essential for the initial cell search in such a system. A solution to this problem is not recited by the conventional technique. If the above described conventional initial search method is sequentially performed for the respective carrier frequencies, in the worst case, the operations at the first through the third stages may be considered to be performed for all of the frequencies. In this case, at least a cell search time multiplied by the number “Nf” (the number of downstream carrier frequencies) of carrier frequencies is required compared with the case of a single carrier frequency.
Additionally, when many mobile stations concentrate on a single cell in the conventional DS-CDMA system, mobile stations exceeding the capacity of one base station attempt to access the station, which can possibly lead to a fault such as a communication quality degradation or communication disability.